In the frame of the OECD international program SERENA, a comparative review of the main computer models and numerics for both the pre-mixing and the explosion phases has been performed. The present paper is a summary of this review and presents the most important models and differences between the FCI codes. These differences concern: the fuel numerical description; the fuel fragmentation; the flow map for multiphase, multi-component fluid systems; the heat and mass transfers between fuel and coolant. These differences enlighten different opinions and partner's views both on physical and numerical aspects.